Deep ultraviolet light sources, such as those used for integrated circuit photolithography manufacturing processes have been almost exclusively the province of excimer gas discharge lasers, particularly KrF excimer lasers at around 248 nm and followed by ArF lasers at 198 nm having been brought into production since the early 90's, with molecular fluorine F2 lasers also having also been proposed at around 157 nm, but as yet not brought into production.
To achieve resolution reduction at a fixed wavelength and fixed numerical aperture (NA) (that is, an 193 nm XLA 165 on a XT:1400 with an NA of 0.93), one must optimize k1, where k1 represents process-dependent factors affecting resolution.
Based on Rayleigh's equation, for dry ArF tools today smaller resolution of state-of-the-art high-numerical-aperture ArF lithography can only be achieved with Resolution-Enhancement Techniques (RET's). RETs are a cost-effective way to maintain the aggressive evolution to smaller dimensions in IC manufacturing and are becoming integral to manufacturing lithography solutions.
These process-related resolution enhancement efforts (lowering k1) have focused on reticle design, using methodologies such as phase shifting or pattern splitting on dual masks. While these techniques improve imaging, they also have significant drawbacks, including throughput loss. So when k1 is optimized for an application the only way to improve resolution further is to go back to the wavelength or NA.
Immersion lithography does just this for the 45 nm, the wavelength is constant at 193 nm so introducing water allows for NA's up to 1.35 and this relaxes the k1 requirement until processing at the 32 nm is required.
Since the first introduction of excimer laser light sources in the DUV, wavelength manufacturers of these light sources have been under constant pressure not only to reduce the wavelength, but also to increase the average power delivered to the wafer in the manufacturing process carried out by steppers and scanners. This requirement for smaller and smaller wavelength has come from the need of the integrated circuit manufacturer customers for the stepper/scanner makers to be able to print smaller and smaller critical dimensions on the integrated circuit wafers. The need for higher power has generally been driven by either the need for more throughput or higher dose for exposing certain photoresists on the wafer, or both.
This steady progression down the road formed by the so-called Moore's law about the progression of integrated circuit capabilities, and thus, basically the number of transistors per unit area and thus also basically smaller and smaller critical dimensions, has created various and serious problems for the light source manufacturers to address. Particularly the move to the 193 nm wavelength node of light sources has resulted in several challenges.
Resolution and critical dimension (CD) control in advanced lithography, at the 193 nm, requires a narrow spectral bandwidth because all lens materials have some degree of chromatic aberration, necessitating a narrow bandwidth laser to reduce the wavelength variation in the light source, thereby diminishing the impact of chromatic aberration. Very narrow bandwidth can improve the ultimate resolution of the system, or, alternatively can give lens designers more focal latitude. However, there is an increase in the impact of speckle on printing more narrow critical dimensions on the wafer.
Partlo et al, in an article entitled “Diffuser speckle model: application to multiple moving diffusers,” Appl. Opt. 32, 3009-3014 (1993), discuss aspects of speckle reduction. U.S. Pat. No. 5,233,460, entitled METHOD AND MEANS FOR REDUCING SPECKLE IN COHERENT LASER PULSES, issued to Partlo et al. on Aug. 3, 1993 discusses misaligned optical delay paths for coherence busting on the output of gas discharge laser systems such as excimer laser systems. Naulleau, Relevance of Mask-Roughness-Induced Printed Line-Edge Roughness in Recent and Future Extreme-Ultraviolet Lithography Tests, Applied Optics, Vol. 43, Issue 20, pp. 4025-4032 (2004) discusses the effects of speckle induced by roughness in the mask coupling to speckle in the arial image of the illuminating light. Lee, Effect of line edge roughness (LER) and line width roughness (LWR) on Sub-100 nm Device Performance, Advances in Resist Technology and Processing XXI, edited by John L. Sturtevant, Proceedings of SPIE Vol. 5376 (SPIE, Bellingham, Wash., 2004) discusses the impacts of photoresist materials and IC manufacturing process parameters on LER/LWR. Rydberg et al., Dynamic laser speckle as a detrimental phenomenon in optical projection lithography, J. Microlith., Microfab., Microsyst. Vol 53, No. 1-1-1-8 (July-September 2006), revision of a paper presented at the SPIE conference on Optical Microlithography XVIII, March 2005, San Jose, Calif., SPIE Proceedings Vol. 5754 refers to the effects of so-called dynamic speckle/coherence on LER/LWR. Yamaguchi et al., Impact of long-period line-edge roughness (LER) on accuracy in CD Measurement, Metrology, Inspection, and Process Control for Microlithography XIX, edited by Richard M. Silver, Proc. of SPIE Vol. 5752 (SPIE, Bellingham, Wash., 2005) discusses the impact of dimension feature size, for example, length on LER. Leunissen et al., Full spectral analysis of line width roughness, Metrology, Inspection, and Process Control for Microlithography XIX, edited by Richard M. Silver, Proc. of SPIE Vol. 5752 (SPIE, Bellingham, Wash., 2005) discusses quantification of LWR. Patsis et al., Integrated simulation of line-edge roughness (LER) effects on sub-65 nm transistor operation: from lithography simulation, to LER metrology, to device operation, Emerging Lithographic Technologies X, edited by Michael J. Lercel, Proc. of SPIE Vol. 6151, 61513J, (2006) discusses LWR metrology and the impacts of LWR on device operating parameters. Pawloski et al., Characterization of line edge roughness in photoresist using an image fading technique, discusses the role of aerial image contrast and image-log-slope (ILS) on the resulting magnitude of line edge roughness (LER) in resist and whether the minimization of LER in current state-of-the-art, chemically amplified resist materials was limited by the quality of the projected aerial image or the materials and processing of the resist and the identification of the iso-fading condition, which in analogy to the iso-focal dose, results in a unique exposure dose for which the critical dimension (CD) of a resist feature does not change with increasing levels of fading. The paper suggests that there is a way to determine undesired iso-fading condition which applicants believe could be used in accordance with aspects of an embodiment of the disclosed subject matter to control coherence busting to attempt to achieve a better or perhaps optimized state of LER on the wafer, and the paper suggests that while the aerial image plays a strong role on determining the magnitude of LER during resist printing, there also exists a fundamental limitation to LER from the resist materials that cannot be improved by further increase in the quality of the aerial image. Cobb et al., EUV photoresist performance results from the VNL and the EUV LLC, Emerging Lithographic Technologies VI, Roxann L. Engelstad, Editor Proceedings of SPIE Vol. 4688 (2002) ©2002 SPIE, discusses the role of optics and masks in LER using EUV light sources. Naulleau, The role of temporal coherence in imaging with extreme ultraviolet lithography optics, Optics Communications 219 (2003) 57-63, ©2003 Elsevier Science B.V., available at www.science direct.com, discusses the impact of temporal coherence in off-axis illumination using EUV broad band light sources. Goodman, Speckle Phenomena in Optics: Theory and Applications, may have information regarding speckle reduction and speckle in photolithography.
As used herein the term resonator and other related terms, such as cavity, oscillation, and output coupler, are used to refer, specifically to either a master oscillator or amplifier portion, a power oscillator, as lasing that occurs by oscillation within the cavity until sufficient pulse intensity exists for a useful pulse to emerge from the partially reflective output coupler as a laser output pulse. This depends on the optical properties of the laser cavity, the size of the cavity, and the reflectivity of the output coupler and not simply on the number of reflections that direct the seed laser input through the gain medium a fixed number of times, for example, a one pass, two pass, etc. power amplifier, or six or so times in the embodiment disclosed in Fork, et al. and not on the operation of some optical switch in the cavity. In some of the literature an oscillator in which the round trip through the amplification gain medium around a loop in a bow-tie or racetrack loop, is not an integer number of wavelengths, may be referred to as an amplifier, for example, a power amplifier, while also constituting an oscillator laser. The term power amplification stage and more specifically ring power amplification stage is intended herein to cover both of these versions of a power oscillator, that is, whether the path through the gain medium is an integer multiple of the laser system nominal center wavelength or not and whether the literature, or some of it, would refer to such an “oscillator” as a power amplifier or not. The closed loop path or oscillation loop as used herein refers to the path through the amplification gain medium such as an excimer or similar gas discharge laser amplification stage, around which the seed laser pulse light oscillates in the amplification stage.
Photolithographic processes for integrated circuit manufacturing processes, for the formation of various integrated circuit structures and patterns, may be used forming patterns by exposing a photoresist (PR) layer. Light is passed through a mask (or reticle), which is comprised of clear areas that transmit the incident light and dark regions that absorb the incident light. An optical projection arrangement forms an image of the mask pattern in the PR layer at a defined plane of the wafer with a certain depth of focus. Areas of the PR layer sufficiently exposed by the light energy, can become either soluble or insoluble upon such exposure, so that desired portions of the PR layer can be removed with a chemical developer solution applied to the surface of the PR layer. The removed areas uncover the underlying layer (such as a semiconductor layer, a metal or metal-containing layer, a dielectric layer, etc.), while the remaining PR protects the underlying layer. The open areas of the underlying layer can subsequently be treated, wet or dry etched, exposed to dopant for assimilation, subjected to ion implantation/etching, etc., while the remaining PR can selectively block areas in the underlying layer where retardation or elimination of the effect of the treatment for the opened areas occurs. The remaining PR film can be stripped thereafter and more PR added and the next layer in the manufacturing process similarly treated.
The ever advancing requirement to increase the density of the patterned areas requires appropriately smaller scaling of the sizes of the imaged PR features and improvement in the resolution of the optical exposure systems used to image the PR layer. As critical dimensions continue to be reduced, the roughness of the edges of the patterned PR structures starts to become noticeable, being transferred into the underlying layer by the accompanying treatment, such as, etch, implantation or reaction processes, and adversely affecting the size and/or shape of features being formed or otherwise treated in the underlying layer. The effects of such feature dimension roughness/variation can be most important in features, such as transistor gates, where precision and uniformity of the edges of particularly the dimensional features defining the width of the gate region that can separate the source and drain regions, and like features critical to performance of the device being made on the integrated circuit. The line-edge roughness (LER) or line-width roughness (LWR) of the PR patterns has particularly been noted in lithography utilizing Argon Fluoride (ArF) excimer laser sources, with exposure wavelengths near 193 nm, with the use of high numerical apertures and/or immersion lithography. Indeed reduction in LER and/or LWR is becoming one of the most important challenges in photolithography for IC fabrication. Therefore, it is highly desirable to reduce as much as possible the dimension roughness/variation of the PR patterns.